1. DATA FLOW DIAGRAMS Learning Units
MODULE 5
DATA FLOW DIAGRAMS
Learning Units
5.1 Developing Data Flow Diagrams(DFD)
a) What are DFDs?
b) Symbols used in DFD
c) Rules of data flow
d) Good style in drawing DFD
5.2 Describing systems with DFD & Levelling DFDs
5.3 Logical & Physical DFDs
System Analysis And Design © V. Rajaraman

2. In this module we will learn What are Data Flow Diagrams (DFDs)?
LEARNING GOALS
In this module we will learn
What are Data Flow Diagrams (DFDs)?
Why they are useful?
How are they developed?
How to level DFDs?
Good style conventions in developing DFDs
Difference between Logical and Physical DFDs
Tools available to draw DFDs
Systems Analysis And Design © V. Rajaraman
1 of 26

3. Provides an overview of -What data a system processes
MOTIVATION
WHY DFD ?
Provides an overview of
-What data a system processes
-What transformations are performed
-What data are stored
-What results are produced and where they flow
Systems Analysis And Design © V. Rajaraman
2 of 26

4. Graphical nature makes it a good communication tool between
MOTIVATION
WHY DFD ?
Graphical nature makes it a good communication tool between
-User and analyst
-Analyst and System designer
Structure of DFD allows starting from a broad overview and expand it to a hierarchy of detailed diagrams
Systems Analysis And Design © V. Rajaraman
3 of 26

5. WHAT ARE DATA FLOW DIAGRAMS?
DFDs models the system by depicting
External entities from which the data flows and where results terminate
Processes which transform data flows
Data stores from which the data are read or into
which data are written by the processes.
5.1.1
Systems Analysis And Design © V. Rajaraman
4 of 26

6. A circle represents a process
SYMBOLS USED IN DFD
PROCESS
Stores demand
note 1.
STORES
Issue Advice
Delivery slip
A circle represents a process
Straight lines with incoming arrows are input data flows
Straight lines with outgoing arrows are output data flows
Processes are given serial numbers for easy reference
Labels are assigned to Data flow.These aid documentation
5.1.2
Systems Analysis And Design © V. Rajaraman
5 of 26

7. A Rectangle represents an external entity
SYMBOLS USED IN DFD
EXTERNAL ENTITIES
Order
Invoice
Customer
VENDOR
Bill
A Rectangle represents an external entity
They either supply data or receive data
They do not process data
5.1.3
Systems Analysis And Design © V. Rajaraman
6 of 26

8. A Data Store is a repository of data
SYMBOLS USED IN DFD
DATA STORES
Inventory
Writing
Reading
A Data Store is a repository of data
Data can be written into the data store
This is depicted by an incoming arrow
Data can be read from a data store
This is depicted by an outgoing arrow
External entity cannot read or write to the data store
Two data stores cannot be connected by a data flow
5.1.4
Systems Analysis And Design © V. Rajaraman
7 of 26

9. -external entity to process -process to external entity
RULES OF DATA FLOW
Data can flow from
-external entity to process
-process to external entity
-process to store and back
-process to process
Data cannot flow from
-external entity to external entity
-external entity to store
-store to external entity
-store to store
5.1.5
Systems Analysis And Design © V. Rajaraman
8 of 26

10. An alternate notation is often used
DATA FLOW DIAGRAMS
An alternate notation is often used 3
Label
A Process
Store
Issue
Name
A Data store
DS1
Inventory
Name
Label
5.1.6
Systems Analysis And Design © V. Rajaraman
9 of 26

11. GOOD STYLE IN DRAWING DFD
Use meaningful names for data flows, processes and data stores.
Use top down development starting from context diagram and successively levelling DFD
Only previously stored data can be read
A process can only transfer input to output.It cannot create new data
Data stores cannot create new data
5.1.7
Systems Analysis And Design © V. Rajaraman
10 of 26

12. DESCRIBING A SYSTEM WITH A DFD
An entire system is represented by one DFD which gives the system’s overview
It is called a context diagram
It gives little detail & is also known as the top level DFD
Context diagram of mess management is shown in the next transparency
5.2.1
Systems Analysis And Design © V. Rajaraman
11 of 26

13. CONTEXT DIAGRAM OF MESS MANAGEMENT SYSTEM
Students
Mess manager
Chief warden
Mess secretary
Vendors
Requisitions
Payments
Daily rate
Menu
Overdue
Item needed
Each day
Perishable
Items
Bills
Extras Note
Supplies
Overdue Bills
Observe this diagram gives very little detail
5.2.2
Systems Analysis And Design © V. Rajaraman
12 of 26

14. A context diagram gives an overview
LEVELLING DFD
A context diagram gives an overview
It should be split into major processes which give greater detail.
Each major process is further split to give more detail.
Each major process is further split to give more detail
5.2.3
Systems Analysis And Design © V. Rajaraman
13 of 26

15. Each DFD must deal with one aspect of a big system
WHY LEVEL DFD?
If a DFD is too detailed it will have too many data flows and will be large and difficult to understand
Start from a broad overview. Expand to details - Idea similar to using procedures and linking these with a main program
Each DFD must deal with one aspect of a big system
5.2.4
Systems Analysis And Design © V. Rajaraman
14 of 26

16. EXPANDED DFD FOR HOSTEL MESS MANAGEMENT1
Billing
system
Students
Mess
Secretary
Chief Warden
Mess manager
Payments
Update daily
rate
Itemized bills at end
of month
Extras/Rebates
Expenses
No of meals
(today +3)
Items used each day
Student billing
Information + bills
Unpaid bills
Going to next process (Continued in next slide)
5.2.5
Systems Analysis And Design © V. Rajaraman
15 of 26

17. EXPANDED DFD FOR HOSTEL MESS MANAGEMENT
Continued 3
Perishable
ordering 2
Stores issue
and
Control
system
Mess
Secretary
Manager
Vendors
Orders
(perishable)
Vendor data
Vegetables and perishable
requisition
Stores
inventory
Order data
Items used
today
Items to be issued
(today +2)
Vendor supplies
Order non-
perishable
Menu
Perishable order
Low stock (today+2)
5.2.6
Systems Analysis And Design © V. Rajaraman
16 of 26

18. EXPANDED DFD-BILLING SYSTEM
Payments
Itemized
bills
1.2
Calculate
Student’s
bills
Unpaid
bills
1.3
Reconcile
payments
Chief
warden
Bills
Mess
Secretary
Students data
Extras/Rebates
Students data
Daily rate average
(upto date)
1.4
Find no
Of meals
to cook
1.1
Calculate
Daily rate
No of meals
(today + 2)
Expenses data
Mess
Manager
Students data
Items rate data
Observe numbering of processes
5.2.7
Systems Analysis And Design © V. Rajaraman
17 of 26

19. Expanded DFD may have data stores
LEVELLING RULES
If process p is expanded, the process at the next level are labeled as p.1,p.2 etc.
All data flow entering or leaving p must also enter or leave its expanded version.
Expanded DFD may have data stores
No external entity can appear in expanded DFD
Keep the number of processes at each level less than 7.
5.2.8
Systems Analysis And Design © V. Rajaraman
18 of 26

20. ILLEGAL CONSTRUCTS IN DFD
No loops are allowed in DFD
A process cannot be a pure decision
Compare
Actual rate > Standard rate
Actual rate <= Standard rate
Actual daily rate
Standard daily rate
A single data flow should not be split into many flows with different labels
No data flow allowed between data stores
5.2.9
Systems Analysis And Design © V. Rajaraman
19 of 26

21. ILLEGAL CONSTRUCTS IN DFD
Get students
extra/rebates
record
Calculate
Bill
Extra/rebate store
Record
Ask for next record
Not correct as loop is formed
5.2.10
Systems Analysis And Design © V. Rajaraman
20 of 26

22. No of meals to be cooked (today +2)
LEVELLING EXAMPLES
Low stock item
(today +2)
No of meals to be cooked (today +2)
Mess manager
Items to be used on (today +2)
Items issued
Low message stock 2
Stores issue
and
control system
Mess secretary
Order for items
Menu for
(Today +2)
Vendor supplies
Order
Vendor
Stores inventory
Vendor
Stores issue control system process
5.2.11
Systems Analysis And Design © V. Rajaraman
21 of 26

23. LEVELLING EXAMPLES 5.2.11 Systems Analysis And Design © V. Rajaraman
Inventory update
And
low stock warning
2.2
Create order
for vendor
Mess manager
Low stock
item
Items used today
Items needed
From 2.3
Stores inventory
Order
Vendor data
Vendor
supplies
Vendor
Order to vendor
2.3
Calculate Items
needed
2.4
Check Item
availability
Mess secretary
Items needed
Low stock items
(today+2)
Menu
(today +2)
No of meals to (today +2)
Stores inventory
5.2.11
Systems Analysis And Design © V. Rajaraman
22 of 26

24. LEVELLING EXAMPLES 5.2.12 Systems Analysis And Design © V. Rajaraman
Top
Level process
Ext B
Ext A 1 2 4 3 F1 F4
1.1
1.2
1.4
1.3
2.1
2.2
2.3
4.3
4.1
4.2
3.1
3.2
3.4
3.3
Process 1
Process 2
Systems Analysis And Design © V. Rajaraman
5.2.12
23 of 26

25. LOGICAL AND PHYSICAL DFD
DFD’S considered so far are called logical DFDs
A physical DFD is similar to a document flow diagram.
It specifies who does the operations specified by the logical DFD
Physical DFD may depict physical movements of the goods
Physical DFDs can be drawn during fact gathering phase of a life cycle
5.3.1
Systems Analysis And Design © V. Rajaraman
24 of 26

26. PHYSICAL DFD FOR ENCASHING CHEQUE
Customer
Clerk
Verify A/C
Signature
Update
Balance
Cashier
Verify Token
Take Signature
Cash
Cheque
Cheque with
Token number
Bad Cheque
Accounts
Store cheques
Entry in
Day Book
Token
5.3.2
Systems Analysis And Design © V. Rajaraman
25 of 26

27. LOGICAL DFD FOR CHEQUE ENCASHMENT
Retrieve
Customer
Record
Check
Balance,
Issue
token
Store
Token no
& cheques
Search
& match
Update
Daily
cash book
Day book
Cash
Token Slip
Cheque with
Cheque
Token
accounts
Cheque store
With token no.
Or cheque
5.3.3
Systems Analysis And Design © V. Rajaraman
26 of 26